Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. To keep with such current trends, a great deal of research and stilly has been focused on development of batteries that can meet various requirements of consumers.
In terms of the battery shape, there is a great demand for rectangular batteries and pouch-type batteries which have a slim thickness and are therefore applicable to electronic products such as mobile phones. In terms of the battery material, there is a great demand for lithium secondary batteries such as lithium cobalt polymer batteries, which have excellent energy density, discharge voltage and safety.
One of main research goals regarding the secondary battery is to improve safety of the battery. Generally, the lithium secondary battery may undergo explosion of the battery die to high internal temperature and pressure thereof which may result from abnormal operation conditions of the battery, such as internal short circuit, excessive charged state over an acceptable level of electric current and voltage, exposure of the battery to high temperatures, and external impact such as falling. For example, the pouch-type secondary battery is susceptible to the risk of an internal short circuit when it is exposed to external impact such as falling or application of force from the outside.
In recent years, rechargeable secondary batteries are widely used as an energy source for wireless mobile equipment. In addition, secondary batteries have also drawn a great deal of attention as a power source for electric vehicles (EVs) and hybrid electric vehicles (HEVs) which are presented as countermeasures capable of solving problems of air pollution caused by fossil-fuel driven vehicles such as gasoline vehicles and diesel vehicles.
Median/large-size devices such as motor vehicles require a high-power, large-capacity power source. For this purpose, a median/large-size battery system with electrical connection of multiple battery cells is typically employed. The pouch-type lithium ion polymer secondary battery, which is widely used as a unit battery in such a middle or large-sized battery system, has a relatively large size, as compared to the same class of other batteries which are used in small-sized devices.
The pouch-type polymer secondary battery is typically comprised of an electrode assembly, electrode tabs extending outward from the electrode assembly, electrode leads welded to the electrode tabs, and a pouch for housing the electrode assembly and formed of a laminated sheet of a polymer resin and aluminum.
Referring to FIG. 1, a lithium ion (polymer) battery 100 has electrode tabs 10,10′ protruding outward therefrom to ensure a flow of electric current. Therefore, sealing of the pouch including the electrode tabs 10,10′ should be made to finish final assembly of the battery. Here, sealing is made on four sides 20a,20b,20c,20d where the pouch and electrode tabs are overlapped with each other. Unfortunately in this case, substantially no sealing is achieved in inside portions 30,30′ of the electrode tabs, so adhesive strength is weak in the region (as indicated by the circle) where the electrode tabs 10,10′ and sealing portions are not overlapped with each other. As a consequence, there is a high possibility of gas discharge toward the region which will have poor adhesive strength upon sealing of the external pouch structure and the electrode tabs.
Such an undesirable event takes place in the configuration of FIG. 2 where the electrode tabs are installed parallel to each other on either side of the battery as well as in the configuration of FIG. 1 where the electrode tabs are installed on both top and bottom sides of the battery.
In a conventional lithium ion (polymer) battery, sealing of the electrode tabs was performed only on electrode tab-pouch overlap portions 20a,20b,20c,20d,120a,120b,120c,120d, as shown in FIGS. 1 and 2.
In such a case, discharge of gases takes place in unexpected regions (as indicated by the circle), which may result in difficulties associated with a design of a battery pack for HEVs. Multidirectional venting of gases from the inside of the battery pack, not unidirectional venting of gases, does not allow easy discharge of gases to the outside. Further, although discharge of internal gases should be achieved more quickly to secure safety of the battery upon evolution of gases inside the battery, a conventional battery is highly vulnerable to significant deterioration of the battery safety due to an extended period of time required for venting gases.
To this end, there is a strong need for development of a secondary battery having a safety device that is capable of preventing rupture or explosion of the battery by blocking evolution of gases in the battery inside through release of high pressure conditions when the battery reaches a given level of internal pressure.